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CorrosionManagement | March/April 2012TECHNICALARTICLEInfluence of Corrosion onDesign of Marine StructuresAli Sarandily, Technical Director, Ports & Marine, AECOM Perth, Western Australia AUSTRALIA.AbstractThis paper is about a study which show howa design life of 50 years for steel marinestructures in a severe marine environment interms of corrosion can be achieved. The studyhas looked at:n Selection of material and form ofconstructionn Corrosion rates in various zonesn Influence of marine environment oncorrosion and mechanism of corrosion;and has concluded that:1. Understanding of the mechanisms ofcorrosion and corrosion issues relating todesign is the first and most important step incorrosion control;2. The occurrence of corrosion in marinestructures is as simple as:A +B =CWhere:A: Aggressive Marine Environment;B: Bad Design/detailing; andC: Concentrated CorrosionThe common factor in all the above is the“prevent the formation of corrosion cellprinciple” which relies on the designerhaving an understanding and awareness ofthe mechanism of corrosion and corrosioncell formation. The designer must considerthe “prevent the formation of corrosion cellprinciple” while formulating steel designdetails for marine structures.Key WordsUniform and concentrated corrosion,durability of marine structures, and marineenvironment.1.0 introductionWhen it comes to corrosion and durabilityof marine structures, there is a belief amongmaritime engineers that corrosion will notoccur and marine structures normally remaindurable if they are fully protected by cathodicprotection or if the steel structure hassufficient corrosion protective coating.18This may be true for uniform and generalcorrosion which occurs in a homogenousmaterial and a member with no differencein electrical potential between any pointson the surface. However, the corrosion rateaccelerates and becomes more concentrateddue to bad design or material choice or acombination of the two, where cathodicprotection will not be effective.This occurs because normally engineers designthe structures and refer to relevant “codesof Practices” 1&2 to cover the corrosion byignoring the principles and basis of corrosionand how and why it occurs. The objective ofthis paper is to look at:n A case study to understand the materialchoice and form of design that is intendedto achieve the design life of the structure;n The basis and forms of corrosion andfactors that affect the corrosivity; andn Provide a general awareness that engineerscould follow to minimize the effect ofcorrosion in marine structures by adopting“prevent the formation of corrosion cellprinciple”.Corrosion occurs on unprotected steelstructures in any location, and varies inintensity depending on local variables 3 butit is the localized and aggressive corrosion inthe marine environment that if it is not dealtwith, may cause the marine structure to fail.The localised corrosion can occur when:n Dissimilar metals are in contact orjoined together; orn A detail exists that allows penetration,retention or trapping of moisture or water;orn A steel member is constantly exposed towetting and drying conditionsMany mistakenly believe that application ofa coating system or cathodic protection willeliminate corrosion but the coating systemhasn’t prevented the corrosion of Delta Pierin and below the Splash Zone where the bondbetween the flame sprayed thermoplasticcoating and the steel sheet pile has failedand re-exposed the structure to the corrosiveenvironment.Engineers often apply time-honored methodsto protect maritime structures againstcorrosion with little or no understanding ofthe principles behind them 5 . The goal of thispaper is to encourage engineers to think moreabout the root cause of the various typesof corrosion in marine structures and applythe “prevent the formation of corrosion cellprinciple” to overcome the occurrence ofcorrosion.The majority of marine structures are madeof carbon steel members or carbon steel (i.e.steel reinforcement) embedded in concretemembers or a combination of the two. They aresubjected to harsh environmental conditionsand loads. The common approach is to designthe structure to resist the environmental loads(i.e. ocean wave, current and tidal actions etc.)but the design for durability comes into 2 ndplace, and if it is overlooked, this may lead tocatastrophic consequences.There are many different technical reasonsfor Marine Structures to fail and failure isnormally a consequence of a combinationof different factors. However failure due tocorrosion could occur if the concentratedcorrosion coincides with the highly stressedbending zone of structural members.There has been no reported catastrophicfailure due to corrosion in operational marinestructures but following highlights the failureof Bridge structures due to Accelerated LowWater Corrosion (ALWC) of piles 6 .An engineering report, investigating thecollapse of a Northern Territory bridge 6 , foundthat two steel piles of the Adelaide RiverBridge were completely corroded throughthe base. This was believed to be caused bya number of factors, including a sulphatereducing bacteria which had attacked thesteel pile and which had not previously beenknown to exist in tropical conditions.For many marine steel structures the issue ofcorrosion is one of uniform or concentratedcorrosion and the design life of marinestructures is normally 50 years and the needto design, detail and construct a durableand a functional structure, requires a detailunderstanding of:n material selection and form of design andits fabrication;
TECHNICALARTICLEn the surrounding environment and itseffect on durability of the structure; andn basis and mechanism of corrosion andhow and when it occurs.All of the above are required so that theintended design life can be achieved byproviding a design solution to corrosionproblems in the marine environment.The reason for selecting this case study is thatthe author was responsible for the design anddesign management of this marine structureand ensured that the durability of structurewas addressed at the design stage and not atthe maintenance stage which is normally toolate and not cost effective.2.0 Case Study2.1 Harriet and NelsonPoint Jetties in PortHedland, AustraliaHarriet and Nelson Point loading Jetties arelocated at Port Hedland (approximately1600 Km from Perth), in Western Australiawhere two new berths will be located ateach jetty with capacity to accommodateCape Class vessels to a maximum of 250,000DWT (Dead Weight Tonne) at each berth.The length of each jetty is 865m and thewidth is approximately 30m (Figure 1). Therequirement was to design an economicalberthing and loading platform that has adesign life of 50 years.Figure 1. Harriet Point Loading Jetty (Image courtesy ofMcConnell Dowell).2.1.1 Selection of Materialand Form of ConstructionThe question of material selection and formof construction is crucial for the long termcorrosion prevention of jetty structures asit is required to withstand the extreme loadcombinations while allowing uninterruptedloading of iron ore to continue. Therefore,a suitable material needs to be robust totransfer the applied load to the foundationsas well as retaining service life for 50 years incorrosive marine environment with minimumpreventive measure without significantmaintenance work.There is really no simple rule for choice ofmaterial in the design of marine structures,it is normally based on the Company ordesigner’s experience on similar projects and:n Cost and their availability;n Design parameters;n Service life;n Short fabrication and installation time;n Applied load; andn Seabed conditionThe Norsok Standard M-001 which iscommonly used for material selection,recommends that the choice of materialshall be such that, general, pitting and crevicecorrosions can cost effectively be prevented 7 .This is true but the fabrication methodand the corrosion rate of the material areequally important as they both influence thecorrosion.The construction material for this type ofstructure is usually a combination of carbonsteel and reinforced concrete members. Themost common Jetty construction consistsof circular carbon steel piles supporting aconcrete or steel frame deck system 8 and inWestern Australia the general approach forthe construction is a modular constructionwith limited site works. This is to prefabricateoffsite a complete section and then transportto site by heavy lift ship or semi-submersiblevessel for final installation. The reason beingthat modular construction provides “highquality” product and fewer defects as it will becarried out under a controlled environment.The construction material for marinestructures differs from country to country.For example the author has experiencedthat concrete marine structures are morecommonly used in Hong Kong and PersianGulf countries than say in United Kingdomand Australia and the reason behind this ispurely economics but the marine environmentof these countries can also be more polluted 9so corrosion prevention of steel structuresin polluted marine environments is not verycost effective.2.1.1.1 Choice of CarbonSteel as a ConstructionMaterial for main framingand substructureCarbon steel has been chosen for the mainframing of superstructures and substructuresof this marine structure. The key driver foruse of carbon steel for the main framing andsubstructure is its versatility to be fabricatedin a form of module offsite and inspectedfor any defects during fabrication includingwelds before installation. This is to identifyand eliminate the sources of corrosion hotspots as well as structural defects. 8&10 Anydefects during fabrication or service lifedue to corrosion either in the member orjoints will later have a detrimental affect onstability of the structure or operation of thefacilities. 3,4&52.1.1.2 ModularConstructionModular construction techniques commonlyused in the oil and gas industry for decades 10are now being successfully applied to some ofthe other large maritime projects undertakenin Australia 8 . After fabrication offshore, wharfmodules are normally transported by heavylift ship to the site, transferred onto theirfinal positions by the ship’s Cranes. Figure 2below shows an example of box girder beamswhere the weld details can easily be inspectedbefore assembly.The initial motivation for using modularconstruction was because of over-ridingrequirements for speed of construction andsite safety when working over water, improvedquality, and for an early return of investment.The same conclusion was also echoed inthe 4 th International Modular Construction& Pre-Assembled Fabrications Conference,recently held in Perth 11 , but they also statedthat the modular construction also providedhigh quality and few defected products withlower cost which is critical for achieving theintended design life of the structure.It has also been proven that use of offsitefabrication is very effective in terms of function,quality, time, cost and operative safety 12 .19
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